More and more, paper is surface treated in order to improve its properties. In the manufacture of printing papers, an aqueous suspension of pigment particles and binder, similar to a latex house paint, is applied at high speeds typically up to 100 km/h with roll or blade coaters. The paper is then dried and calendered. The finished surface is very smooth, uniform and glossy and gives excellent print quality particularly in multicolour printing.
The binders currently used in pigmented coatings are soluble polymers such as starch or polyvinyl alcohol or latexes based on styrenebutadiene, polyvinyl acetate or acrylic monomers. The synthetic latex binders are not biodegradable. Starch is biodegradable but has poor binding efficiency and is water-sensitive, a problem in multicolour offset printing where water is applied at each station, as well as in water-based gravure or flexographic printing.
The market for pigmented coated papers is growing extremely fast, currently about 10-15% per year, and it may be predicted that even the lower grades such as newsprint will be coated one way or another in the near future.
Surface treatments and impregnations are applied for reasons other than that of improving print quality. The size-press, and its newer versions, are used to apply a number of chemicals that will change the surface and also the bulk properties of paper. Hydrophobicity, mechanical strength, grease resistance, release properties, friction, flammability, electrical conductivity are examples of properties that can be imparted to paper by such means.
In packaging grades, hydrophobicity, papers that are non-wetting, water fast and a property called "stiffness-when-wet" are requirements that are difficult to meet adequately. Usually, petroleum-based resins and waxes are used to obtain the effect, but it is difficult to reach high levels of hydrophobicity and wet-stiffness without imparting also a high level of permanency to the product.
The recycling of paper and paper fibers is increasingly demanded by an environmentally sensitive public as well as for economic reasons. Up to now, there has been a lack of sensitivity in the design of paper coating thermoplastics for ultimate recycling and currently coated paper is not readily recycled.
Synthetic latices are employed as binders in fibre constructs including paper and non-woven fabrics, and disposal of such fibre constructs also represents an environmental problem.
Self-supporting polymer films are used in a variety of applications, for example, water impermeable films in disposable diapers, and these films too, represent an environmental problem.
Self-supporting films which are biocompatible are also employed for controlled delivery of drugs in the body. Such films may be microporous with the drug being held within the micropores, or the film may form a capsule about the drug, and degrade slowly in the body to release the drug. It is important that the degradation products of such films be non-toxic in the body.
Thermoplastic polymers of bacterial origin are known which behave like polypropylene and related polyolefins, but which are biodegradable. These polymers form a family of poly-.beta.-hydroxyalkanoates based mainly on .beta.-hydroxyalkanoic acids containing 4 or 5 carbon atoms and are, more especially, isotactic crystalline polyesters of high molecular weight.
These poly-.beta.-hydroxyalkanoates and in particular poly-.beta.-hydroxy-butyrate (PHB) and (.beta.-hydroxybutyrate-.beta.-hydroxyvalerate) (PHB/V) copolymer are microbially produced in nature and can be synthesised on an industrial scale from a variety of bacteria, by fermentation technology. These polymers are in powder available commercially in a spray dried particle form.